[#1502] Use zap.Error for logging errors

Use `zap.Error` instead of `zap.String` for logging errors: change all expressions like
`zap.String("error", err.Error())` or `zap.String("err", err.Error())` to `zap.Error(err)`.
Leave similar expressions with other messages unchanged, for example,
`zap.String("last_error", lastErr.Error())` or `zap.String("reason", ctx.Err().Error())`.

This change was made by applying the following patch:
```diff
@@
var err expression
@@
-zap.String("error", err.Error())
+zap.Error(err)

@@
var err expression
@@
-zap.String("err", err.Error())
+zap.Error(err)
```

Signed-off-by: Aleksey Savchuk <a.savchuk@yadro.com>

pkg/core/object/sender_classifier.go

@@ -67,7 +67,7 @@ func (c SenderClassifier) IsInnerRingOrContainerNode(ctx context.Context, ownerK
 	if err != nil {
 		// do not throw error, try best case matching
 		c.log.Debug(ctx, logs.V2CantCheckIfRequestFromInnerRing,
-			zap.String("error", err.Error()))
+			zap.Error(err))
 	} else if isInnerRingNode {
 		return &ClassifyResult{
 			Role: acl.RoleInnerRing,
@@ -84,7 +84,7 @@ func (c SenderClassifier) IsInnerRingOrContainerNode(ctx context.Context, ownerK
 		// is not possible for previous epoch, so
 		// do not throw error, try best case matching
 		c.log.Debug(ctx, logs.V2CantCheckIfRequestFromContainerNode,
-			zap.String("error", err.Error()))
+			zap.Error(err))
 	} else if isContainerNode {
 		return &ClassifyResult{
 			Role: acl.RoleContainer,